1. Field of the Invention
The present invention relates to a display control system and method, and more particularly to a display control system and method for controlling a display such as a ferroelectric liquid crystal display of the type having display elements capable of holding the data by applying an electric field or the like.
2. Related Background Art
Information processing systems have a display as a means for visually displaying information. As such displays, a CRT display is widely used.
For the control of a CRT display, the data write operation by a system CPU into a video memory as a display data buffer and the data read operation by a CRT controller from the video memory are executed independently.
With such control of a CRT display, both the data read/write operations are independent so that a program used by the information processing system is not required to consider the display timings at all, allowing to write the display data at an optional timing.
The screen of a CRT display has a certain degree of depth, increasing the dimension of the CRT display and hindering the compactness of it. The information processing system using a CRT display has therefore a poor degree of freedom with respect to the installation site, portability and the like.
A liquid crystal display (hereinafter called LCD) solves such problems, and provides the compactness (particularly, small depth of the display screen). Of various types of LCDs, there is a display using ferroelectric liquid crystal (FLC) cells (hereinafter called FLCD (FLC display)). One of the main features of FLCD is a function of holding the display data upon application of an electric field to the liquid crystal cells. Namely, FLCD has liquid crystal cells so thin that the elongated FLC molecules are oriented in the first or second stable direction depending upon the electric field application direction, maintaining the orientation even after the electric field is removed. Because of such bi-stable nature of FLC molecules, FLCD provides a memory function. The details of FLC and FLCD are described, for example, in Japanese Patent Application No. 62-76357 (U.S. Ser. No. 174,980 filed on Mar. 29, 1988).
Different from a CRT display or other liquid crystal displays, there is therefore a marginal time for the period of the refresh operation sequentially executed for FLCD, and in addition it is possible to execute a partial rewrite of updating only the changed display data on the display screen.
According to one approach to using such features, there is known a method wherein flags are prepared as many as the number of scan lines of the display screen, and when a data rewrite occurs in a VRAM, the corresponding flag is set to preferentially rewrite the scan line with the set flag.
It is necessary to control the display if the rewrite time of the scan line depends on a temperature. The display control, such as controlling the period of the refresh operation and the number of scan lines to be interlaced during the interlacing operation, is executed depending upon a change in the temperature.
The above conventional method requires to check the flags as many times as the number of scan lines of the display screen. It takes a lot of time to check the flags in the case of a high resolution display (for example, 1280 dots.times.1024 dots of screen size), lowering the throughput of the display control.
Furthermore, because the scan line rewrite time depends on the temperature of a display and becomes long as the temperature lowers, the number of scan lines to be interlaced and the field frequency are increased to suppress flickers. In this case, the rewrite operation cannot be executed at high speed.
Interlacing has been used conventionally for displays such as CRT and LCD displays. This interlacing is used when the refresh rate is low. The quality of an image sequentially displayed on the display screen at the low refresh rate is degraded by image disturbance called flickers.
Interlacing is used to suppress this image disturbance phenomenon. As shown in FIG. 34, after displaying line 0, not the line 1 but the line delayed by several lines from line 0 is displayed. In the interlacing operation shown in FIG. 34, every fourth lines are displayed, namely, in the order of line 0, line 4, line 8, line 12, line 16, line 20, line 1, line 5, and etc. By displaying lines at an interval of some lines, an apparent display speed rises, preventing the image disturbance called flickers.
For a display such as an FLCD having a small number of display colors, a binarization process such as a dither process may sometimes become necessary. The binarization process is performed generally in units of a plurality of lines.
However, the above-described interlacing operation displays at an interval of some lines. Therefore, in the binarization process such as a dither process, complicated processing is required such as the preparation of buffers for the dither process, thereby lowering the processing speed and image quality and leading to high cost.
With high speed data rewrite (such as character scrolling), the above-described interlacing operation may cause fluttering of characters, degrading the image quality of characters.
In the case where a certain display unit such as a character line is rewritten by a FLCD, if the display unit is different from the partial rewrite unit, there is a time difference between partial rewrites, lowering the image quality to "fluttered image" display. Furthermore, a partial rewrite is executed even for unnecessary lines, lowering the processing speed.